YSM Issue 90.4

FEATURE
geology and geophysics
ARCTIC OCEAN CRATERS
Sudden methane release linked to ice sheet retreat
►BY THEO KUHN
In the depths of the arctic Barents Sea, north of the northernmost
point of Norway, the seafloor is pocked with large craters and
mounds. These craters don’t have their origins in fire, but in ice. Researchers
believe that they formed due to the sudden breakdown
of gas hydrate—an ice-like substance found both in seafloor sediments
and on land, which can rapidly transition from a solid to
a gas. New research led by Karin Andreassen at the University of
Norway suggests that these craters released great expulsions of gas
at the end of the last Ice Age. Furthermore, the study concludes that
receding ice sheets were a primary factor in drawing these gases—
which mostly consist of methane, a powerful greenhouse gas—out
of the seafloor and potentially releasing them into the atmosphere.
Gas hydrates are substances that form when hydrocarbon gases
interact with water and sediment under the appropriate conditions
and crystallize. Though they are relatively simple compounds,
the factors that control their stability—determining whether they
remain solid or decompose into gas and liquid water—are surprisingly
complex. The most important factors are pressure and temperature;
the pressure must be high and the temperature must be
low for the hydrate to remain solid.
To develop a model for the origin of the craters, Andreassen’s
team combined their observations of the seafloor with a chemical
analysis of the hydrates and a model of the physical conditions that
would have existed in their study area over the past 30,000 years.
Their findings paint a picture in which crater formation was the direct
result of deglaciation.
Near the end of the last Ice Age, which occurred 17,000 years
ago, more than a kilometer of solid ice lay on the seafloor at the
study location. Underneath it, there was a large zone with the optimal
conditions for methane hydrate formation. But within two
thousand years, that ice sheet receded from the area due to the
warming climate. “It happened quite fast,” Andreassen said.
The loss of such a large mass of ice caused a sudden change in
pressure, and, to a lesser extent, temperature, that shrank the zone
in which methane hydrates were stable. As methane hydrates became
unstable and decomposed, the newly-released gas collected
underneath a shrinking cap of solid hydrate. Eventually, the high
concentration of gas and hydrate in a dwindling area caused the
seafloor to distend and fracture, further lowering the pressure and
allowing the remaining hydrates to suddenly decompose. The
scars of this runaway process are the craters that remain to this
day.
Andreassen’s model is especially intriguing because it doesn’t
just connect the sudden decay of gas hydrates to ice sheets; it also
suggests that glaciation helps to form large quantities of gas hydrates
in the first place. Methane typically leaks towards the surface
from hydrocarbon deposits that lie in the bedrock far beneath
the surface—the types of formations that can be drilled into for natural
gas extraction. As an ice sheet advances over these area, its immense
weight forces gas upwards and towards the surface where it
can form gas hydrates, and as ice sheets recede, much of this hydrate
then decomposes. “It’s like a big bulldozer going back and forth,”
Andreassen said. “The ice will pump the gas up from the deeper
reservoirs and into the shallow surface, where it can form gas hydrates.”
Methane is 25 times more potent than carbon dioxide as a greenhouse
gas. Methane is currently seeping from the seafloor, but most
of it doesn’t reach the atmosphere—its slow release rate allows seawater
to absorb it before it breaches the surface. In contrast, the
events that formed the craters are believed to have occurred abruptly.
Thus, these large quantities of gas released may have been able to
bubble to the surface and enter into the atmosphere.
Researchers hope to find traces of these events in atmospheric
records that will help them determine if the events contributed to
the change in climate that occurred at the end of the last Ice Age. In
the short term, Andreassen’s team hopes to understand the extent
of gas hydrates and craters across the Arctic Ocean, since large areas
have experienced a similar glacial history and may have undergone
the same process of hydrate production. By developing a clearer
picture of the history of gas hydrates in Arctic areas, researchers
may be able to predict how a similar process might unfold in the
future—to find ice sheets that are quickly receding, one may need
not look back fifteen millennia.
IMAGE COURTESY OF WIKIMEDIA COMMONS
► Methane hydrate from the seafloor near Oregon. Methane
hydrate can be found within sediments and sedimentary rocks,
primarily within the continental shelves of the world’s oceans.
26 Yale Scientific Magazine October 2017 www.yalescientific.org